Personal identity

What is it to be the same person today as one was in the past, or will be in the future? How are we to describe cases in which (as we might put it) one person becomes two? What, if anything, do the answers to such questions show about the rationality of the importance we attach to personal identity? Is identity really the justifier of the special concern which we have for ourselves in the future? These are the concerns of this entry.

In order to answer the question about the persistence-conditions of persons we must indulge in some thought experiments. Only thus can we tease apart the strands that compose our concept of personal identity, and thereby come to appreciate the relative importance of each strand. There are plausible arguments against attempts to see the relation of personal identity as constitutively determined by the physical relations of same body, or same brain. I can survive with a new body, and a new brain. But it does not follow; nor is it true, that a person’s identity over time can be analysed exclusively in terms of psychological relations (relations of memory, belief, character, and so on). To the contrary, the most plausible view appears to be a mixed view, according to which personal identity has to be understood in terms of both physical and psychological relations. This is the view which can be extracted from our core (that is, minimally controversial) set of common-sense beliefs about personal identity.

The possibility of the fission of persons– the possibility that, for example, a person’s brain hemispheres might be divided and transplanted into two new bodies – shows that the mixed view has to incorporate a non-branching or uniqueness clause in its analysis. The concept of personal identity, contrary to what we might first be inclined to believe, is an extrinsic concept (that is, whether a given person exists can depend upon the existence of another, causally unrelated, person).

Some philosophers have recently tried to forge an important connection between theories of personal identity and value theory (ethics and rationality). The possibility of such a connection had not previously been investigated in any detail. It has been argued that, on the correct theory of personal identity, it is not identity that matters but the preservation of psychological relations such as memory and character. These relations can hold between one earlier person and two or more later persons. They can also hold to varying degrees (for example, I can acquire a more or less different character over a period of years). This view of what matters has implications for certain theories of punishment. A now reformed criminal may deserve less or no punishment for the crimes of their earlier criminal self. Discussions of personal identity have also provided a new perspective on the debate between utilitarianism and its critics.